Hi John,
Let me give you a brief context explanation. A new DSP is coming out called the Helix Ultra. I was a bit disappointed that it doesn't come with FIR filters which I found to be very useful since it retains phase information. Another well-known member goes on to tell me, you don't need FIR filters since, in a mostly minimum phase environment like a car, you could just match the magnitude of the frequency response to match its phase response. I clearly thought what he said is wrong. I'm now on a quest to find out if this is true.
Here's what was said on another forum.
Me:
Hey Nick, I'm pretty sure that's wrong. You matching the EQ on each side does not put your system's response "in phase". The EQ you're performing on each side is most likely different in order to center the stereo image. The effect you're hearing of the center image being centered is just one part of the phantom image illusion. That one part is amplitude, the other is the phase.
Another forum member:
And from what I understand, this is wrong (edit: maybe "not correct for our in car use" is the right way to put it). A cars sound system is MOSTLY minimum phase since the reflections are so early that they tend to not behave differently than ones in a larger room. You can test this by cutting a peak by x db. If it drops by x db, its minimum phase. If you boost by x db and the response goes up by x db, its minimum phase. In a minimum phase system, fixing the frequency response = fixing the phase response = fixing the impulse response. It's easily measurable.
Comparing a room to a cars interior isnt anywhere near apple's to apple's. The reason a cars system can act as a quasi-minimum phase system is because most of the reflections are to early.
As far as helix not adding fir filters, they have the capability and I asked julian and florian last week when we were in Dallas flat out why they haven't put them in yet. They went into great detail as to why with the above being the super watered down version. Just not really as good in a car as people make it seem and I've had that theory of my own for about 2 years now.
I'm pretty sure what he is saying is incorrect but I just want to get a second opinion from someone who is more knowledgable than me. I'm not trying to win an argument but really just understand what he is saying exactly. It doesn't make any sense to me.
When using an IIR filter and you're applying different EQ curves to both side, how in the world would the IIR filter (which causes phase shift in varying amounts depending on the amounts or cuts or boosts) find its way back to the correct phase at the listening position? What he is saying doesn't make sense to me so maybe some of you could make sense of it for me. Mainly the stuff underlined above.
I'm in the process of testing FIR filters in a car and I found that when the phase information is preserved I was able to hear much more detail in the sound stage and the transients are much sharper. When I switch my DSP over to IIR filters using the same EQ curves, I lose all the realism and everything collapses to a semi-smeary image. Both IIR and FIR frequency response curve closely matches since I had my processor do the FIR to IIR conversion (it was an option they offered). So I'm pretty sure that the phase retention feature of an FIR filter helps a lot compared to the IIR counterpart.
Basically, John, what I'm asking is... Does matching the magnitude of the left and the right speakers also match the phase response when using an IIR filter? I think what you said is, NO it doesn't. The magnitude of an IIR filter is linked to its phase response so if you were to EQ each side differently you will in effect cause phase distortion that doesn't match which will most likely result in a smearing effect. Correct?
Let's say I have the example below:
Let's pretend that I apply the EQ curves below to the L and the R side:
Then let's pretend that now both the L and R side are now matching with 1dB tolerance. The question now is the phase response of the L and R side now matched? Simply by just matching its magnitude. My brain says NO. But that other forum member is saying that in a minimum phase system like a car, the phase response will match as long as you match the magnitude.
Thanks in advance John.